Ceramic honeycomb structural body and methods of preparing the same

ABSTRACT

The present invention provides a ceramic honeycomb structural body, and method for preparing the same, which can satisfactorily support a catalyst and can demonstrate superior mechanical strength, even if far thinner walls are used. Said structural body comprises lattice walls formed by a large number of cells  10  which are channels for a fluid and a peripheral wall  12  which covers at least the circumference of said lattice walls  11.    
     The outer peripheral portion  112  of said lattice walls  11  located in the vicinity of said peripheral wall  12  (area B) contain a densified portion of smaller porosity than that of an inner peripheral portion  111 , of said lattice walls  11 , which is located inside said outer peripheral portion  112  (area A).

BACKGROUND OF THE INVENTION

[0001] 1. Fields of the Invention

[0002] This invention relates to a ceramic honeycomb structural body tobe used for, e.g., a catalyst support for purifying exhaust gases of aninternal combustion engine, or a filter for purifying water, etc., andto methods of preparing the same.

[0003] 2. Description of Related Art

[0004] In recent years, with the appearance of stricter emissionregulations regarding automobile engines, it has been required toactivate a catalyst for purifying exhaust gases early in order to reducehydrocarbon emission just after starting the engines. To lower thethermal capacity of the ceramic honeycomb structural body havingcatalyst components is one of means used to provide such earlyactivation of the catalyst. It is required to reduce the thickness ofcell walls of said ceramic honeycomb structural body when such means areused. However, the mechanical strength of said structural body decreaseswith a decrease in the thickness of the cell walls of said structuralbody.

[0005] With respect to means for improvement of the mechanical strengthof the honeycomb structural body, it has been proposed, as described inJapanese Publication No. 62-6855, to fill pores in walls, in theneighborhood of an outer peripheral wall among the outer peripheral walland lattice walls consisting of channel cells, with reinforcingmaterials of a lower porosity.

[0006] However, the honeycomb structural body reinforced by theabove-mentioned method has a problem with lower durability.Specifically, when a ceramic honeycomb structural body is located, as acatalyst support for a catalyst for purifying exhaust gases, in theexhaust pipe of engine, said body is exposed repeatedly to thermalchanges from a low temperature to a high temperature. Therefore, when itis used, it sometimes suffers delamination between the lattice walls andthe reinforcing materials, or a thermal shock fracture due to thethermal history. Furthermore, said delamination and fracture may ariseduring the cooling process after the firing process for manufacturingsaid body. It is supposed that such in conveniences result from thedifference, in thermal expansion coefficients, between said latticewalls and said reinforcing materials, and the discontinuity betweenreinforced and non-reinforced areas.

[0007] In addition, as mentioned above, when pores are filled withreinforcing materials, the disappearance of pores open to the surfaceleads to a decrease in the surface area of said body. Therefore, theamount of a supported catalyst or the amount of a supported carrierhaving large surface area, which is a medium for applying a catalyst tosaid body, decreases, and then it is difficult to satisfactorily achievecatalytic performance. Further, when pores are filled with reinforcingmaterials, alkali metals or alkaline earth metals, such as K, Na, Caetc., are added as a flux component so as to improve the bondabilitywith a matrix. However, these materials cause an increase in the thermalexpansion coefficient, and then when thermal changes are repeated, theuse of the reinforcing materials is apt to result in heat shock fractureor delamination.

SUMMARY OF THE INVENTION

[0008] In light of the foregoing circumstances, it is an object of thepresent invention to provide a ceramic honeycomb structural body, andmethods of preparing the same, which can satisfactorily support acatalyst and can demonstrate superior mechanical strength, even if farthinner walls are used.

[0009] The above object is achieved according to a first embodiment ofthe present invention by providing a ceramic honeycomb structural body,comprising: lattice walls formed by a large number of cells which arechannels for a fluid; and a peripheral wall which covers thecircumference of said lattice walls, an outer peripheral portion of saidlattice walls at least located in the vicinity of said peripheral wallcontaining a denser portion of a lower porosity than that of an innerperipheral portion of said lattice walls which are located inside saidouter peripheral portion.

[0010] Other and further object, features and advantages of the presentinvention will appear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The invention will be more particularly described with referenceto the accompanying drawings, in which:

[0012]FIG. 1 is a perspective view of an embodiment of the invention;and

[0013]FIG. 2 is a enlarged view of details of the place M taken fromFIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

[0014] The invention provides a ceramic honeycomb structural body,comprising: lattice walls formed by a large number of cells which arechannels for fluid; and a peripheral wall which covers the circumferenceof said lattice walls, an outer peripheral portion of said latticewalls, at least located in the vicinity of said peripheral wall,containing a denser portion of a lower porosity than that of an innerperipheral portion of said lattice walls which is located inside saidouter peripheral portion.

[0015] It should be noted that, in the invention, the outer peripheralportion itself of said lattice walls is made denser without usingconventional reinforcing materials, at least said outer peripheralportion of said lattice walls having a smaller porosity than said innerperipheral portion. Therefore, said outer peripheral portion becomes areinforced portion which has a higher mechanical strength than saidinner peripheral portion of a larger porosity. Thus, when said latticewalls and said peripheral wall are made thinner to lower the thermalcapacity of a ceramic honeycomb structural body, the mechanical strengthrequired of said entire body is maintained due to the presence of saidreinforced portion at least in said outer peripheral portion. On theother hand, said inner peripheral portion can have a high porosity asusual.

[0016] Further, as mentioned above, the mechanical strength of saidouter peripheral portion is improved by lowering the porosity itself,and reinforcing materials do not fill the pores. Thus, pores open to thesurface of said outer peripheral portion can be maintained, and anuneven surface and a large surface area can be secured. Thereby, whensaid ceramic honeycomb structural body is used as a catalyst support,the amount of the supported catalyst or carrier can be sufficientlysecured. Consequently, according to the invention, the ceramic honeycombstructural body can satisfactorily support a catalyst and candemonstrate superior mechanical strength, even if far thinner walls areused.

[0017] In a preferred embodiment of the invention, it is preferable thatΔPr demonstrated by the following equation is not less than 5%:

ΔPr=−(Pout−Pin)/Pin

[0018] in which Pout is the porosity of said denser portion in the outerperipheral portion of said lattice walls, and Pin is the porosity of theinner peripheral portion of said lattice walls. When said ΔPr is lessthan 5%, there is a problem that said densified portion cannot have asufficient mechanical strength.

[0019] Furthermore, preferably, the thickness of the outer peripheralportion of said lattice walls is not less than 1.2% of a distancebetween the center of said body and the inner side of said peripheralwall. When said thickness is less than 1.2%, there is a problem thatsaid structural body cannot have sufficient mechanical strength.Moreover, it is preferable that the porosity of said denser portion inthe outer peripheral portion of said lattice walls is gradually reducedfrom the inside to the outside. This constitution can restrain rapidchanges of the mechanical strength in the boundary area between theinner and the outer peripheral portions, and can reduce the stressconcentration to the boundary area.

[0020] In another preferred embodiment of the invention, it ispreferable that the thickness of the outer peripheral portion of saidlattice walls is 0-400% larger than that of said inner peripheralportion. As a means for strengthening of said outer peripheral portion,this embodiment may be used along with said densification. On the otherhand, when the thickness of the outer peripheral portion is thinner thanthat of said inner peripheral portion (said figure is less than 0%),there is a possibility that the effect of an improvement of mechanicalstrength, due to increased density, may decrease thereby. In addition,when the thickness of the outer peripheral portion is in excess of fivetimes that of said inner peripheral portion (said figure is more than400%), there is a possibility that a pressure drop may increase as afluid passes through.

[0021] Further, it is also preferable that the boundary area between theinner and the outer peripheral portions in said lattice walls graduallydecreases, in thickness, from outside to inside. This constitution canrestrain rapid changes of the shape in the boundary area between theinner and the outer peripheral portions, and can reduce the stressconcentration in the boundary area.

[0022] Furthermore, the difference between the inner and the outerperipheral portions in said lattice walls is preferably within±0.5×10⁻⁶/° C. When said difference is not in this range, there is aproblem that said structural body may be prone to thermal shockfracture, due to heat history, with large changes in temperature.

[0023] Furthermore, another embodiment of the invention provides amethod of preparing the ceramic honeycomb structural body, comprising:lattice walls formed by a large number of cells which are channels forfluid and a peripheral wall which covers the circumference of saidlattice walls, the outer peripheral portion of said lattice walls atleast located in the vicinity of said peripheral wall containing thedenser portion of smaller porosity than that of the inner peripheralportion of said lattice walls which are located inside said outerperipheral portion, characterized in that a melting-point loweringcomponent, which lowers a melting point of a material constituting saidceramic honeycomb structural body, is applied on at least the outerperipheral portion of said lattice walls of said body, and then theresulting body is heat-treated to form said denser portion.

[0024] It should be noted that, in this method, said melting-pointlowering component is applied to the desired portion, and the resultingbody is heat-treated to change the partition wall itself into saiddenser portion.

[0025] Said melting point lowering component may be properly chosenaccording to the materials constituting said structural body. Forexample, when said body comprises a cordierite, a component which canlower the melting point of the cordierite by increasing the content issuitable. Talc, alumina, kaolin or a mixture thereof, which onconstituents of cordierite, may be suitably used as said melting pointlowering component. Also, a contaminant constituent of said cordierite,such as iron or titanium, may be used as said component. The increase insaid contaminant content lowers the melting point of the cordierite.

[0026] Further, in order to apply said melting-point lowering componentto said outer peripheral portion of said lattice walls, for example, amixed solution of said melting point lowering component and a solventmay be applied on the desired portion. Said solvent includes varioussolvents such as an aqueous solvent (water, etc.) or a non-aqueousorganic solvent.

[0027] Also, various methods, such as dipping, spraying, may be adoptedto apply said mixed solution to said structural body. Said body to beapplied with said melting point lowering component may be a body whichhas been dried after extrusion, or as fired after drying.

[0028] In the method of the invention, after said melting-point loweringcomponent is applied to the outer peripheral portion of said latticewalls of said body, the resulting body is heat-treated. Thus, theportion where said melting-point lowering component is applied decreasesin its porosity and forms a denser portion. Therefore, said outerperipheral portion of said lattice walls becomes a reinforced portionwhich has a higher mechanical strength than said inner peripheralportion.

[0029] Such mechanisms may be as follows:

[0030] When said firing is performed, the melting point of a portionwhich is contacts said melting point lowering component decreases, andthe portion is partially fused according to the firing temperature.Thereby, such a fused portion penetrates the insides of the pores andfills said pores. Thus, a portion where said melting-point loweringcomponent is applied on decreases in porosity, forms a dense portion andincreases in a mechanical strength. Therefore, according to theinvention, said structural body having said denser portion can be easilyprepared.

[0031] Subsequently, in an embodiment of the invention, the content ofalkali metals and alkaline earth metals, such as K, Na and Ca, in saidmelting-point lowering component is preferably less than 0.5%. Thisconstitution can restrain a increase in a thermal expansion coefficientof said lattice walls. When said content of alkali metals and alkalineearth metals is more than 0.5%, there is a problem that the resultingdenser portion may suffer an increase in the thermal expansioncoefficient to be prone to thermal shock fracture.

[0032] Also, in another embodiment of the invention, said melting-pointlowering component is preferably applied on the outer peripheral portionof said lattice walls in a manner which the applying amount graduallyincreases from inside to outside. In this case, said lattice walls canhave a changing density, that is the outer portion is denser than theinner portion. Thereby, the stress concentration can be restrained and aremarkably improved durability can be obtained.

[0033] Further, the invention provides a ceramic honeycomb structuralcatalyst, wherein a catalyst component is supported by said ceramichoneycomb structural body.

[0034] Also, the invention provides a method of preparing a ceramichoneycomb structural catalyst, wherein a catalyst component is supportedby said ceramic honeycomb structural body.

[0035] As the ceramic honeycomb structural catalyst according to theinvention has said superior structural body, it makes it possible toactivate the catalyst early and it has excellent durability.

EXAMPLES

[0036] Referring to FIGS. 1 and 2, there is shown a preferred embodimentof a ceramic honeycomb structural body and methods of preparing the sameaccording to the invention.

[0037] These examples show a ceramic honeycomb structural body to beused for a catalyst support for purifying exhaust gasses, the ceramicmaterial being a cordierite.

[0038] The ceramic honeycomb structural body, as shown in FIGS. 1 and 2,consists of lattice walls 11 formed by a large number of cells 10, whichare channels for a fluid, and a peripheral wall 12 which covers thecircumference of said lattice walls 11. As shown in FIG. 2, a peripheralwall 12 as well as an outer peripheral portion 112 of said lattice walls11 located in the vicinity of said peripheral wall 12 (area B) contain adensified portion of a smaller porosity than that of an inner peripheralportion 111 of said lattice walls 11 which is located inside said outerperipheral portion 112 (area A).

[0039] A cordierite which constitutes said structural body has atheoretical composition of 2MgO.2Al₂O₃.5SiO₂. The composition usuallyincludes 49.0-53.0 wt % of SiO₂, 33.0-37.0 wt % of Al₂O₃ and 11.5-15.5wt % of MgO. When a cordierite honeycomb structural body is molded, talc(Mg₃Si₄O₁₀(OH)₂), kaolin (Al₂Si₂O₅(OH)₄), alumina (Al₂O₃) and so on areusually used as raw materials for a cordierite. Various compounds otherthan these compounds may be properly used as sources of Si, Al or Mg.For example, said compounds include oxide, nitride, carbide, boride,hydroxide and chloride, etc. which contain at least one of the elementsSi, Al and Mg.

[0040] In these examples, talc, kaolin, alumina and aluminium hydroxidepowder were used as raw materials for preparing a cordierite and mixedtogether at a rate such that the composition after firing was in theneighborhood of the theoretical one. To 100 wt % of said raw materials,2.8 wt % of a lubricant and a wetting agent, 5.5 wt % of a binder and anadequate amount of water were added and kneaded to obtain a body. Thelubricant and the wetting agent were a 5% solution of polyalkyleneglycol and the binder was methyl cellulose (water-soluble).Subsequently, said body was molded by extrusion to form a honeycombshape and then dried. The molded body was cylindrical in shape. Thesizes were 100 mm in outer diameter of the peripheral wall 12, 100 mm inlength, 0.3 mm in the thickness of the peripheral wall 12 and 60 μm inthe thickness of the lattice walls 11.

[0041] Then, a melting-point lowering component, which lowers a meltingpoint of a material constituting said ceramic honeycomb structural body,is applied on said peripheral wall and said outer peripheral portion ofsaid lattice walls of said structural body, and then the resulting bodyis heat-treated. In this example, talc, which is a source of Si and. Mg,is used as a melting-point lowering component. Application to saidstructural body was carried out using a solution in which said talc isdispersed in a non-aqueous solution.

[0042] Specifically, said structural body was dipped in said solution,the longitudinal direction of the cells being kept horizontal. Saiddipped structural body was rotated. Thereby, said solution was appliedon the whole B area in FIG. 1, that is said peripheral wall and saidouter peripheral portion of said lattice walls.

[0043] Then, any extra solution obstructing the fluid channels waseliminated by an air blower and the body was drid and then heat-treated.The heat-treatment was carried out at a temperature which was not lessthan the melting point of said applied portion and less than that of thecordierite composition. Firing was performed at 1430° C. for 4 hours inthe atmospheric. Thus, the area B, comprising a peripheral wall 12 aswell as an outer peripheral portion 112 of said lattice walls 11, had adenser portion of a smaller porosity than that of the area A comprisingan inner peripheral portion 111.

[0044] Subsequently, the properties of the resultant honeycombstructural body 1 were measured for said area A and area B as well asfor the entire body. A comparative body was prepared by the same methodas these examples, except for the application of said melting-pointlowering component, and its properties were also determined.

[0045] The obtained results are shown in Table 1. The porosity in Table1 was determined by using a porosimeter. The thermal expansioncoefficient was determined by measuring in the range from roomtemperature to 800° C. through the use of a thermal expansioncoefficient measuring device. The A-axis compression strength wasdetermined by using an autographic recorder. Further, the isostaticstrength was determined by an isostatic strength measuring device. TABLE1 Example Comparative body (with densification) (without densification)Properties A area B area Entire A area B area Entire Porosity 35 30 — 3535 35 (%) Thermal 0.5 0.5 0.5 0.5 0.5 0.5 expansion coefficient (×10⁻⁶/°C.) A-axis 5 12 — 5 6 — compression strength (Mpa) Isostatic — — 2.5 — —1.7 strength (Mpa)

[0046] It should be understood that there is no difference in thethermal expansion coefficient between the area A and the area B, but thearea B is denser and has a porosity which is 5% less than that of thearea A. Therefore, the A-axis compression strength of the area B isabout twice that of the area A, and the isostatic strength showing theentire strength is 50% more than that of the comparative body.

[0047] Thus, in these examples, the peripheral wall 12 and the outerperipheral portion 112 are made denser to have a smaller porosity.Therefore, the area B, comprising the peripheral wall 12 as well as theouter peripheral portion 112, becomes a reinforced portion which hashigher mechanical strength than said inner peripheral portion of largerporosity. Thus, as mentioned above, even if the thickness of saidlattice walls 11 is reduced to 60 μm, the mechanical strength requiredof said entire body is maintained due to the presence of said reinforcedportion in said peripheral wall 12 and said outer peripheral portion 11.

[0048] Further, the mechanical strength of said peripheral wall and saidouter peripheral portion is improved by forming the denser portion, andreinforcing materials do not fill the pores. Thus, the pores open to thesurface of said outer peripheral portion can be maintained, and a unevensurface and a large surface area can be secured. Thereby, the amount ofthe supported catalyst or carrier can be sufficiently secured. Thus, thecatalytic effects can be significantly improved owing to thesatisfactory support and the early activation.

[0049] In these examples, talc was used as a melting-point loweringcomponent. However, the degree of density may be properly changed byselecting parameters such as types or particle sizes of raw materials,solvents, concentration of solutions, and firing temperature, etc.Specifically, as is apparent from a phase diagram (not shown), variouscompounds other than talc may be used as a melting-point loweringcomponent to have a same effect as the talc as follows:

[0050] The compounds may, for example include Si—Mg source such asserpentinite (Mg₃Si₂O₅(OH)₄), chlorite and forsterite (Mg₂SiO₄), Sisource such as kaolin, pyrophyllite (Al₂Si₄O₁₀(OH)₂), silica stone(SiO₂) and silicon dioxide, as well as Mg source such as brucite(Mg(OH)₂), magnesium hydroxide and magnesium oxide.

[0051] Further, when the ceramic is a cordierite; of which the thermalexpansion coefficient results from its crystalline orientation, as insaid examples, it is more desirable to use anisotropic particles, suchas lamellar, needle-like or columnar particles of talc, kaolin and soon, than isotropic particles, to maintain a lower thermal expansioncoefficient.

[0052] Also, said melting-point lowering component may be applied byvarious methods other than the above-mentioned method. For example, atthe end of said structural body, after the area corresponding to theinner peripheral portion 111 of said lattice walls is covered with ablind patch, a solution containing said lowering component may besprayed longitudinally along the channels. Furthermore, the solution maybe also sprayed along a ringed guide or diffuser just corresponding tosaid area B.

[0053] Also, although the examples describe a cordierite honeycombstructural body to be used for a catalyst support for purifying exhaustgases of an internal combustion engine, the ceramic honeycomb structuralbody according to the invention can be used for other application, suchas a filter for purifying water, etc.

[0054] While the invention has been particularly shown and describedwith reference to the preferred embodiments thereof, it will beunderstood by those skilled in the art that the foregoing and otherchanges in form and details can be made therein without departing fromthe spirit and scope of the invention.

What is claimed is:
 1. A ceramic honeycomb structural body, comprising:lattice walls formed by a large number of cells which are channels forfluid; and a peripheral wall which covers the circumference of saidlattice walls, an outer peripheral portion of said lattice walls atleast located in the vicinity of said peripheral wall containing adenser portion of a smaller porosity than that of an inner peripheralportion of said lattice walls which is located inside said outerperipheral portion.
 2. The ceramic honeycomb structural body, as claimedin claim 1, wherein ΔPr demonstrated by the following equation is notless than 5%: ΔPr=−(Pout−Pin)/Pin in which Pout is a porosity of saiddenser portion in the outer peripheral portion of said lattice walls,and Pin is a porosity of the inner peripheral portion of said latticewalls.
 3. The ceramic honeycomb structural body, as claimed in claim 1or 2, wherein a thickness of the outer peripheral portion of saidlattice walls is not less than 1.2% of a distance between the center ofsaid body and the inner side of said peripheral wall.
 4. The ceramichoneycomb structural body, as claimed in claim 1, 2 or 3, wherein theporosity of said denser portion in the outer peripheral portion of saidlattice walls is gradually reduced from the inside to the outside. 5.The ceramic honeycomb structural body, as claimed in one of claims 1-4,wherein the thickness of the outer peripheral portion of said latticewalls is 0-400% larger than that of said inner peripheral portion. 6.The ceramic honeycomb structural body, as claimed in any one of claims1-5, wherein a boundary area between the inner and the outer peripheralportions in said lattice walls gradually decreases in thickness from theoutside to the inside.
 7. The ceramic honeycomb structural body, asclaimed in any one of claims 1-6, wherein the difference in thermalexpansion coefficient between the inner and the outer peripheralportions in said lattice walls is within ±0.5×10⁻⁶/° C.
 8. A method ofpreparing the ceramic honeycomb structural body, comprising: latticewalls formed by a large number of cells which are passes for fluid; andthe peripheral wall which covers the circumference of said latticewalls, the outer peripheral portion of said lattice walls at leastlocated in the vicinity of said peripheral wall containing the portionof smaller porosity denser than that of the inner peripheral portion ofsaid lattice walls which are located inside said outer peripheralportion, characterized in that a melting-point lowering component, whichlowers a melting point of a material constituting said ceramic honeycombstructural body, is applied on at least the outer peripheral portion ofsaid lattice walls of said body, and then the resulting body isheat-treated to form said denser portion.
 9. The method of preparing theceramic honeycomb structural body, as claimed in claim 8, wherein thecontent of alkali metals and alkaline earth metals in said melting-pointlowering component is less than 0.5%.
 10. The method of preparing theceramic honeycomb structural body, as claimed in claim 8 or 9, whereinsaid melting-point lowering component is applied on the outer peripheralportion of said lattice walls in a manner in which the applying amountgradually increases from the inside to the outside.
 11. A ceramichoneycomb structural catalyst, wherein a catalyst component is supportedon the ceramic honeycomb structural body, as claimed in any one ofclaims 1-7.
 12. A method, for preparing a ceramic honeycomb structuralcatalyst, wherein a catalyst component is supported on the ceramichoneycomb structural body, as claimed in any one of claims 1-7.